Powerline IS a great solutoin, but if your question is "how do I get
the signal to reach the top of my house", then consider the antenna
side of things first.

Consider that your antenna probably has a donut- shaped pattern and
depending on location and position your upstairs may be in the "hole",
not the donut. Turn the antenna toward the horizontal to send the
signal upward and see if that helps.

See if you can move your router/AP upstairs or to the middle of the
house and again, experiment with different positions for the antenna.

Consider a higher gain omni antenna. Radio Shack has 'em. It's
stronger, but the donut gets flatter, more like a disc as the gain
goes up.

Or you could use a panel antenna from one end of the house and point
it to cover the rest of the house.

Steve